Scientists have designed a novel super-capacitor that could help develop cell phones that recharge their batteries themselves in seconds and work for weeks.

The super-capacitor designed by material scientists at Vanderbilt University is the first made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers.

It should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings, researchers said.

"If you ask experts about making a super-capacitor out of silicon, they will tell you it is a crazy idea. But we've found an easy way to do it," said Cary Pint, assistant professor of mechanical engineering who headed the study.

Instead of storing energy in chemical reactions the way batteries do, "supercaps" store electricity by assembling ions on the surface of a porous material.

As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries.

Pint and his colleagues decided to take a radically different approach: using porous silicon, a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer.

This allowed them to create surfaces with optimal nanostructures for super-capacitor electrodes, but it left them with a major problem.

Silicon is generally considered unsuitable for use in super-capacitors because it reacts readily with some of chemicals in the electrolytes that provide the ions that store the electrical charge.

With experience in growing carbon nanostructures, Pint's group decided to try to coat the porous silicon surface with carbon.

When the researchers pulled the porous silicon out of the furnace, they found that it had turned from orange to purple or black.

After they inspected it under a powerful scanning electron microscope they found that it looked nearly identical to the original material but it was coated by a layer of graphene a few nanometres thick.

The researchers tested the coated material and found that it had chemically stabilised the silicon surface.

When they used it to make super-capacitors, they found that the graphene coating improved energy densities by over two orders of magnitude compared to those made from uncoated porous silicon and significantly better than commercial super-capacitors.